In recent years, there have appeared portable personal computers (hereinafter referred to as portable PCs) developed in various sizes and provided with various functions so as to cope with the spread of mobile computing. For example, there are lap-top personal computers, more compact lap-top personal computers, palm-top personal computers, PDA (Personal Data Assistants) devices, etc.
A portable PC includes a built-in battery which allows use of the portable PC in an environment in which no commercial electric power source is available, for example, in a train. Generally, a rechargeable battery that can be charged for repetitive use is employed as such a battery described above.
While rechargeable batteries are employed widely in home electric appliances including portable PCs, etc., an intelligent battery with an electronic circuit integrated therein is now attracting a great deal of attention. According to this intelligent battery, the electronic circuit integrated in the battery can communicate the battery residual capacity to the connected external device. Consequently, for example, where such an intelligent battery is employed in a portable PC, the user can know the residual capacity of the battery before it is used up during an operation in an environment in which no commercial electric power source is available. The user can thus prevent the portable PC from being shut down abruptly during the operation.
Generally, such an intelligent battery stores capacity information denoting its total capacity beforehand so as to obtain the residual capacity by subtracting the discharged capacity from the total capacity denoted in the capacity information. The discharged capacity is obtained by integrating the discharged current value of the battery.
However, such batteries generally have the characteristic that the total capacity is reduced due to the repetition of charging and discharging. FIG. 10 shows an example of the cycle characteristics of an existing battery configured by three lithium-ion batteries (rated voltage: 4.2V) connected serially under temperatures of 20° C. and 40° C. respectively. In FIG. 10, it is premised that both charging current and discharging current are 2.5A.
As shown in FIG. 10, the greater the number of cycles, the lower the total battery capacity. This tendency appears more strongly under higher ambient temperatures. The “number of cycles” mentioned here is the number of discharging times continued until the battery capacity reaches 0% after the battery is charged from 0% to 100% in capacity.
While the cycle characteristics shown in FIG. 10 are an example of those for the lithium-ion battery, the same tendency also appears in other batteries such as the nickel-hydrogen battery, the nickel-cadmium battery, etc.
This is why ordinary intelligent batteries learn the capacity information by replacing the capacity information with the discharged capacity respectively as shown in FIG. 11. In this case, it is assumed that the discharged capacity of the battery at that time is an accurate total battery capacity when the battery capacity reaches zero or a predetermined capacity near zero. Consequently, it is possible to improve the accuracy of the residual capacity to be obtained after that.
However, the above technique for enabling the battery to learn the capacity information when the capacity reaches zero or a predetermined capacity near zero does not always work; in which case, the accuracy of the residual capacity cannot be improved. This problem arises due to the following two reasons.
First, it is generally very rare that a second battery is discharged completely before it is recharged. As shown in FIG. 11, the battery is often charged when it is only partially discharged to a certain capacity. This cycle operation is repeated. And, in case such the cyclical operation is continued, the battery never learns the capacity information.
Second, “Windows98” of Microsoft Corp., USA, which is an operating system adopted in many personal computers (PC), enables the user to set the percentage of the total capacity of the second battery to be used. The default capacity for this setting is 3%. In this case, the system goes into the hibernation or standby state when the residual capacity of the battery reaches 3% even when the user wants to use the battery up to its zero capacity. Consequently, the battery is never used until its capacity reaches zero in any actual use, so that the battery does not learn the capacity information.
In the above case, the capacity information of the battery is kept at the default capacity (the capacity of the new battery) set in the factory even when the battery is used for a long time and the actual total capacity is reduced. In case the user begins using the battery in such a state, the battery might cause a residual capacity skip error (for example, the residual capacity is reduced to 10% abruptly from 50%). In this case, the user will judge the battery abnormal, so the maker is often forced to replace the battery.
Under such circumstances, it is an object of the present invention to provide an electric power unit, an electric power capacity information compensator, and an electric power capacity information compensating method that can compensate battery capacity information more accurately, as well as a computer that can more accurately compensate the capacity information of its built-in electric power unit.